Production of ceramic filaments

ABSTRACT

Apparatus for producing refractory material filaments comprises an enclosure (11), means for heating the constituents of the refractory material, non-contacting means to support a drop (17) of molten material such as silicon, being one of the constituents of the refractory material within the enclosure and means to transport a filament (10) such as carbon, being a second constituent of the refractory material, through the drop of molten material in use. A levitation coil and/or an inert gas jet is used to support the drop of molten material and a silica glass or other refractory tube (13) is used to direct the gas jet. The enclosure has opposed side arms (15, 16) respectively for entry and exit of the filament (10) or the filament may be passed vertically through the molten silicon drop. An inclined side tube may be provided through which grains of silicon may be introduced into the enclosure for addition to the molten drop. In one form, initial heating of the silicon prior to levitation is achieved by providing a resistively heated silicon nitride platform on which the silicon is melted, the platform then being moved from the field of the levitation coil to leave a levitated silicon drop.

BACKGROUND OF THE INVENTION

The invention relates to refractory fibers and in particular but notexclusively to the production of silicon carbide and silicon carbidecoated carbon fibers.

DISCUSSION OF PRIOR ART

GB Patent No 2,227,483 describes a method of manufacture of refractoryfibers of silicon carbide or silicon carbide coated carbon by passingfine carbon fibers through molten silicon. Molten silicon is containedwithin a crucible provided with one or more holes in its base throughwhich fiber(s) can be drawn. The holes are dimensioned such that surfacetension forces in the molten silicon prevent silicon from flowing out ofthe crucible.

The object of the present invention is to provide apparatus and methodfor manufacture of refractory filaments, particularly silicon carbide,utilizing molten material such as silicon without problems associatedwith crucible containment.

The invention provides in one form apparatus for producing refractorymaterial filaments comprising: an enclosure; means for heating theconstituents of the refractory material; non-contacting means to supporta drop, of molten material, being one of the constituents of therefractory material, within the enclosure; and means to transport afilament made of material, being a second constituent of the refractorymaterial, through the drop of molten material in use.

In one arrangement the apparatus may be used with a carbon filament: andmolten silicon.

The means to support or levitate the silicon drop may be a levitationcoil and/or a gas jet. A silica glass or other refractory tube may beprovided to direct the gas jet and the gas may be selected to provide aninert, protective atmosphere to prevent oxidation.

Advantageously the enclosure includes opposed side arms which arerespectively for entry and exit of the filament. Alternatively thefilament may be passed vertically through the molten silicon drop. Aninclined side tube may be provided through which grains of silicon maybe introduced into the enclosure for addition to the molten drop.

In one form, initial heating of the silicon prior to levitation may beachieved by providing a resistively heated silicon nitride platform onwhich the silicon can be melted, the platform being removable from thefield of the levitation coil so as to leave a levitated silicon drop.Alternatively a laser may be provided to melt the silicon.

The invention also provides in a second form a method of making siliconcarbide filaments comprising the steps of:

a) forming a molten drop of silicon;

b) levitating the molten drop; and

c) passing a carbon filament through the molten drop.

The molten drop may be formed by:

a) placing silicon on a platform;

b) heating the platform; then

c) removing the platform.

In one form the platform is made of silicon nitride such that it is notwetted by molten silicon. The silicon can then be melted by resistivelyheating the platform or heating by means of a laser.

Once the silicon is melted the means to levitate the molten silicon maybe applied and the platform removed to leave the levitated drop ofsilicon.

The levitation may be achieved by application of an electromagneticfield from a levitation coil or by forming an upward jet of supportinggas.

Preferably the gas is selected such that oxidation of the molten silicondoes not occur.

The carbon filament may conveniently be passed horizontally through themolten drop.

Preferably the molten drop of silicon is replenished by addition ofsolid silicon to the molten drop to thereby enable continuousmanufacture of silicon carbide filaments.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention will now be described with reference to the accompanyingFigure which shows a carbon filament 10 being fed into a reactionenclosure 11. The reaction enclosure 11 is formed by a vertical silicatube 12, open at its lower end 13 and closed at its upper end 14. Twoopposed lateral side tubes 15, 16 are provided through which the carbonfilament 10 is fed from a take-off spool (not shown) to a take-up spool(not shown). As shown, a spherical drop 17 of molten silicon islevitated by means of an external levitation coil (not shown forclarity) such that the carbon fiber 10 passes through the moltensilicon. An inert gas is introduced into the silica tube 13 to preventoxidation of the heated silica such that the gas enters the open end 13of the tube and flows out through the side tubes 15 and 16.

Silicon will not normally levitate when cold because of its low roomtemperature electrical conductivity. To overcome this problem a siliconnitride platform 18 can be provided to support solid silicon within thelevitation coil and within the heated region in the silica tube 12. Thesilicon nitride and the silicon are platform then heated resistively oralternatively the silicon is heated directly by laser 19 and once thesilicon is red hot, the platform is slowly moved downwards through thetube 12 while the levitation coil is energized. Silicon nitride is aparticularly suitable material for the platform as it is not wetted bythe silicon.

In use one or more carbon filaments are drawn through the moltensilicon, the filament diameter and drawing speed determining whether:the carbon is coated with silicon; the carbon surface is converted tosilicon carbide; or the carbon is completely converted to siliconcarbide.

The upward flow of gas through the reaction enclosure 11 assists insupporting the molten silicon drop 17 and it may be possible to dispensewith the levitation coil and make use of the gas lift alone. In analternative arrangement the side tubes 15 and 16 may be dispensed withand the filament may be drawn upwardly through the molten silicon dropand through a suitable orifice at the top of the silica tube 12.

Although described in relation to silicon carbide filament manufacturethe invention may be applied to any filamentary material which is to becoated or reacted with a further material which is capable of beinglevitated within a reaction enclosure.

A further sloping side tube 20 (as shown in dashed lines) may beprovided in the wall of the silica tube such that grains of silicon canbe introduced therethrough so as to fall into the molten silicon drop toreplenish lost silicon.

We claim:
 1. Apparatus for producing refractory material filaments (10)comprising:an enclosure (11); means for heating constituents of therefractory material; non-contacting means for supporting a drop (17) ofmolten material, comprising a first constituent of the refractorymaterial, within the enclosure, said non-contacting means compriseselectromagnetic levitation; and means for transporting a filamentcomprising a second constituent of the refractory material, through thedrop of molten material.
 2. Apparatus for producing refractory materialfilaments as claimed in claim 1 wherein the filament (10) is carbon andthe molten material (17) is silicon.
 3. Apparatus for producingrefractory material filaments as claimed in claim 1 wherein a silicaglass tube (13) comprises a means for providing an inert gas in saidenclosure.
 4. Apparatus for producing refractory material filaments asclaimed in claim 1 wherein the enclosure includes opposed side arms(15,16) which are respectively for entry and exit of the filament (10).5. Apparatus for producing refractory material filaments as claimed inclaim 1 wherein the filament (10) is passed vertically through themolten drop (17).
 6. Apparatus for producing refractory materialfilaments as claimed in claim 1 further including an inclined side tubecomprising a means for introducing grains of silicon into the enclosurefor addition to the molten drop (17).
 7. Apparatus for producingrefractory material filaments as claimed in claim 2 further including aresistively heated silicon nitride platform on which the silicon can bemelted prior to levitation, the platform being located in a position inthe field of a levitation coil and removable therefrom so as to leave alevitated silicon drop (17).
 8. Apparatus for producing refractorymaterial filaments as claimed in claim 1 further including for meltingthe silicon.
 9. A method of making silicon carbide filaments by passinga carbon filament through molten silicon, said method comprising thesteps of:a) forming a molten drop (17) of silicon; b) levitating themolten drop; and c) passing a carbon filament (10) through the moltendrop (17) so as to form said silicon carbide filament.
 10. A method ofmaking silicon carbide filaments as claimed in claim 9 wherein themolten drop (17) is formed by the steps of:a) placing silicon on aplatform; b) heating the platform until said silicon is molten; then c)removing the platform.
 11. A method of making silicon carbide filamentsas claimed in claim 10 wherein said levitating the molten drop (17)comprises the step of application of an electromagnetic field from alevitation coil.
 12. A method of making silicon carbide filaments asclaimed in claim 10 further including, after the drop forming step andbefore the filament passing step, the step of flowing an inert gasaround said drop such that oxidation of the molten silicon does notoccur.
 13. A method of making silicon carbide filaments as claimed inclaim 10 wherein the carbon filament (10) is passed horizontally throughthe molten drop (17).
 14. A method of making silicon carbide filamentsas claimed in claim 10 further including the step of replenishing themolten drop (17) of silicon by addition of solid silicon to the moltendrop to thereby enable continuous manufacture of silicon carbidefilaments.
 15. A method of making silicon carbide filaments as claimedin claim 12 further including, after the drop forming step and beforethe filament passing step, the step of flowing an inert gas around saiddrop such that oxidation of the molten silicon (17) does not occur.